1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a gettering technique. In particular, the present invention relates to a method of manufacturing a semiconductor device using a semiconductor film having an amorphous structure formed by adding a metallic element, which promotes a crystallization of the semiconductor film.
Note that, in the present specification, the term xe2x80x9csemiconductor devicexe2x80x9d indicates a category of general devices which are capable of functioning by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic equipments are all included in the category of semiconductor devices.
2. Description of the Related Art
Thin film transistors (hereinafter referred to as TFTs) are known as typical semiconductor elements that use semiconductor films having a crystalline structure. TFTs are attracting attention as a technique of forming an integrated circuit on a glass or other insulating substrate, and devices utilizing TFTs, such as a liquid crystal display device with a built-in driving circuit, are beginning to appear on the market. In the conventional art, a semiconductor film with a crystalline structure is formed by using heat treatment or laser annealing to crystallize an amorphous semiconductor film that is obtained by deposition through plasma CVD or reduced pressure CVD. (Laser annealing is the technique of crystallizing a semiconductor film through irradiation of laser light.)
The thus formed semiconductor film with a crystalline structure is a mass of crystal grains. Since the crystal grains are randomly oriented and the orientation thereof cannot be controlled, the semiconductor film affects TFT characteristics. A Japanese Patent Application Laid-Open No. 07-183540 discloses a technique to tackle this problem. The technique involves doping with a metallic element that accelerates crystallization of a semiconductor film, such as nickel, to form a semiconductor film having a crystalline structure. The technique can cause a large proportion of crystal grains to orient in the same direction, and can lower the heating temperature required for crystallization as well. When this semiconductor film having a crystalline structure is used in a TFT, the field effect mobility is improved and the sub-threshold coefficient (S value) is reduced to improve the electric characteristics of the TFT exponentially.
By using a metallic element for promoting crystallization, generation of nuclei in crystallization can be controlled. Therefore, film quality thus obtained is uniform in comparison with another crystallization method in which nuclei are generated at random, and ideally, it is desirable that metallic elements are completely removed or reduced to an allowable range. On the other hand, the metallic element used in doping for accelerating crystallization remains in the semiconductor film having a crystalline structure, or on the surface thereof, causing problems such as fluctuation in characteristic of semiconductor elements obtained. For example, the remaining metallic element increases OFF current in the TFTs to cause fluctuation between the individual elements. In short, the metallic element for accelerating crystallization becomes an unwanted presence once the semiconductor film having a crystalline structure is formed.
Gettering using phosphorus is actively employed as an effective method of removing a metallic element that accelerates crystallization from a specific region of a semiconductor film having a crystalline structure. For instance, the metallic element can readily be removed from a channel forming region by doping a source/drain region of a TFT with phosphorus and subjecting the film to heat treatment at 450 to 700xc2x0 C.
Phosphorus is implanted in a semiconductor film having a crystal structure by ion doping (a method of dissociating PH3 or the like by plasma and accelerating the ions with an electric field to implant the ions in a semiconductor which basically does not include ion separation). The phosphorus concentration necessary for gettering is 1xc3x971020/cm3 or more. Phosphorus doping by ion doping makes a semiconductor film having a crystal structure amorphous. An increase in phosphorus concentration is a problem because it inhibits later recrystallization by annealing. Another problem is that high concentration of phosphorus doping prolongs treatment time needed for the doping and lowers the throughput in the doping step.
To invert the conductivity type of source and drain regions of a p-channel TFT which have been doped with phosphorus, the concentration of boron required is 1.5 to 3 times the phosphorus concentration. This not only makes recrystallization difficult but also increases the resistance of the source and drain regions.
If gettering is not thorough and the degree of gettering fluctuates throughout the substrate, it causes a slight difference, namely, fluctuation in each TFT characteristic. When TFTs arranged in a pixel portion fluctuate in electric characteristic in a transmissive liquid crystal display device, the level of voltage applied fluctuates between pixel electrodes. This leads to fluctuation in amount of light transmitted, which is recognized by a viewer as display irregularity.
A TFT is an element indispensable to a light emitting device using an OLED when the device is driven by an active matrix driving method. Therefore a light emitting device using an OLED has in each pixel at least a TFT that functions as a switching element and a TFT for supplying a current to an OLED. The luminance of a pixel is determined by ON current (Ion) of a TFT that is electrically connected to an OLED to supply a current to the OLED irrespective of the circuit structure and driving method of the pixel. Therefore, in the case of all-blank display, for example, variation in ON current results in fluctuation in luminance.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a measure to solve those problems, namely, a technique of using a metal element that accelerates crystallization of a semiconductor film to obtain a semiconductor film with a crystal structure and then removing the metal element remaining in the film effectively.
The technique of gettering is deemed as a major one in manufacture techniques of an integrated circuit using a single crystal silicon wafer. Gettering utilizes some energy to make metal impurities that have been introduced into a semiconductor segregate in a gettering site in order to reduce the impurity concentration in an active region of an element. There are roughly two types of gettering, extrinsic gettering and intrinsic gettering. Extrinsic gettering obtains the gettering effect by an external strain field or chemical action. Phosphorus gettering, in which a high concentration of phosphorus diffuses into a semiconductor from the back side of a single crystal silicon wafer, corresponds to extrinsic gettering. The above-described gettering using phosphorus too is one of extrinsic gettering.
On the other hand, intrinsic gettering utilizes a strain field of a lattice defect where oxygen generated inside a single crystal silicon wafer plays a part. The present invention focuses attention on this intrinsic gettering utilizing lattice defect or lattice strain and employs the following measures which are applied to about a 10 to 100 nm-thick semiconductor film having a crystal structure.
The present invention has a step of forming on a silicon nitride film a first semiconductor film having a crystal structure by using a metal element, a step of forming a film that serves as an etching stopper (barrier layer), a step of forming a second semiconductor film containing a noble gas element (gettering site), a step of gettering to move a metal element to the gettering site, a step of removing the second semiconductor film, and a step of removing the barrier layer.
The present invention is characterized in that, in the step of forming the film that serves as an etching stopper (barrier layer), a thin silicon oxynitride film is formed by plasma CVD. The material gas of the barrier layer are silane-based gas (monosilane, disilane, trisilane, and the like) and nitrogen oxide-based gas (gas expressed as NOx). For example, a combination of monosilane (SiH4) and nitrous oxide (N2O), or a combination of TEOS gas and N2O, or a combination of TEOS gas, N2O, and O2 is used as material gas to form a silicon oxynitride film with a thickness of 10 nm or less, preferably 5 nm or less. This silicon oxynitride film adheres to the first semiconductor film having a crystal structure better than an oxide film obtained from an aqueous solution containing ozone (typically ozone water) (called chemical oxide), or an oxide film obtained by oxidizing the surface of the first semiconductor film having a crystal structure using ozone generated by ultraviolet irradiation in an oxygen atmosphere. Therefore the silicon oxynitride film is not peeled off in the subsequent step (the step of forming the second semiconductor film). In order to enhance the adhesion of the film even more, argon plasma treatment may be conducted before the barrier layer is formed. The silicon oxynitride film in the thickness range given in the above allows the metal element to pass the barrier layer and move to the gettering site in the gettering step. The selective ratio of the silicon oxynitride film to the second semiconductor film is high as well as the selective ratio of the silicon oxynitride film to the first semiconductor film. Therefore the silicon oxynitride film is very effective as an etching stopper when removing the films after gettering.
In the step of forming the second semiconductor film that serves as a gettering site in the present invention, a semiconductor film containing a high concentration of noble gas element and having an amorphous structure, typically, an amorphous silicon film, is formed by plasma CVD from monosilane, a noble gas element, and hydrogen as material gas, or monosilane, a noble gas element, and nitrogen as material gas. Disilane or trisilane may be used instead of monosilane.
Alternatively, the second semiconductor film that serves as a gettering site may be a semiconductor film containing phosphorus or noble gas and having an amorphous structure which is formed by plasma CVD from monosilane, phosphine (PH3), and a noble gas element as material gas, or monosilane, phosphine (PH3), and hydrogen as material gas, or monosilane, phosphine (PH3), and nitrogen as material gas.
Invention Structure 1 regarding a manufacture method disclosed in this specification is a method of manufacturing a semiconductor device, comprising:
a first step of forming on an insulating surface a first semiconductor film having an amorphous structure;
a second step of doping the first semiconductor film having an amorphous structure with a metal element;
a third step of crystallizing the first semiconductor film to form a first semiconductor film having a crystal structure;
a fourth step of forming a barrier layer on the first semiconductor film having a crystal structure;
a fifth step of forming a second semiconductor film on the barrier layer;
a sixth step of gettering to move the metal element into the second semiconductor film and to remove or reduce the metal element in the first semiconductor film having a crystal structure;
a seventh step of removing the second semiconductor film; and
an eighth step of removing the barrier layer.
In the above Structure 1, a method of manufacturing a semiconductor device is characterized in that the barrier layer is a silicon oxynitride film with a thickness of 1 to 10 nm. The silicon oxynitride film is formed by plasma CVD in which silane-based gas and nitrogen oxide-based gas are introduced as material gas into a film forming chamber to generate plasma.
In the above Structure 1, the fourth step and the fifth step can be carried out using plasma CVD without exposing the device to the air and the throughput is improved.
The second semiconductor film that serves as a gettering site may be a laminate consisting of two or more layers and formed by plasma CVD from different kinds of material gas given in the above. To give an example of the laminate, a semiconductor film containing noble gas and having an amorphous structure is formed from monosilane, a noble gas element, and nitrogen as material gas, and then a semiconductor film containing noble gas and having an amorphous structure is formed from monosilane, a noble gas element, and hydrogen as material gas and layered on the former semiconductor film. When this laminate structure is employed, the upper layer (third semiconductor film), namely, the semiconductor film formed from monosilane, a noble gas element, and hydrogen as material gas to contain noble gas and have an amorphous structure, can contain a higher concentration of noble gas than the lower layer (second semiconductor film), namely, the semiconductor film formed from monosilane, a noble gas element, and nitrogen as material gas to contain noble gas and have an amorphous structure. The laminate structure thus can raise the gettering efficiency and therefore is preferable. The adhesion of the third semiconductor film to the silicon oxynitride film is relatively weak, but the second semiconductor film improves the adhesion between the two films so that the third semiconductor film is not peeled off. In order to enhance the adhesion even more, argon plasma treatment may be conducted before the third semiconductor film is formed. Preferably, the second semiconductor film is thinner than the third semiconductor film since the upper layer (third semiconductor film), namely, the semiconductor film formed from monosilane, a noble gas element, and hydrogen as material gas to contain noble gas and have an amorphous structure, is etched more easily than the lower layer (second semiconductor film), namely, the semiconductor film formed from monosilane, a noble gas element, and nitrogen as material gas to contain noble gas and have an amorphous structure when removing the gettering site.
Invention Structure 2 regarding a manufacture method disclosed in this specification is a method of manufacturing a semiconductor device, characterized by comprising:
a first step of forming on an insulating surface a first semiconductor film having an amorphous structure;
a second step of doping the first semiconductor film having an amorphous structure with a metal element;
a third step of crystallizing the first semiconductor film to form a first semiconductor film having a crystal structure;
a fourth step of forming a barrier layer on the first semiconductor film having a crystal structure;
a fifth step of forming a second semiconductor film on the barrier layer, the second semiconductor film containing a noble gas element;
a sixth step of forming a third semiconductor film on the second semiconductor film, the third semiconductor film containing a noble gas element in a concentration higher than the noble gas element concentration in the second semiconductor film;
a seventh step of gettering to move the metal element into the second semiconductor film and the third semiconductor film and to remove or reduce the metal element in the first semiconductor film having a crystal structure;
an eighth step of removing the second semiconductor film and the third semiconductor film; and
a ninth step of removing the barrier layer.
In the above Structure 2, the third semiconductor film may be formed by plasma CVD in which at least monosilane and noble gas are introduced as material gas into a film forming chamber to generate plasma, or by plasma CVD in which at least monosilane, noble gas, and hydrogen are introduced as material gas into a film forming chamber to generate plasma.
The second semiconductor film that serves as a gettering site may have a noble gas element concentration gradient for efficient gettering. In this case, the noble gas element concentration in the second semiconductor film may be graded by adjusting film formation conditions (including the RF power, film formation pressure, gas flow rate, and the like). When the second semiconductor film has a noble gas element concentration gradient, it makes easier for the metal element that has been moved by gettering to a lower part of the second semiconductor film to travel toward the surface where the concentration is high, and it prevents the film""s ability of gettering the metal element from reaching saturation. When a certain amount of metal element is moved by gettering to the second semiconductor film containing a noble gas element, saturation is reached and the metal element is gettered no more.
Invention Structure 3 regarding a manufacture method disclosed in this specification is a method of manufacturing a semiconductor device, characterized by comprising:
a first step of forming on an insulating surface a first semiconductor film having an amorphous structure;
a second step of doping the first semiconductor film having an amorphous structure with a metal element;
a third step of crystallizing the first semiconductor film to form a first semiconductor film having a crystal structure;
a fourth step of forming a barrier layer on the first semiconductor film having a crystal structure;
a fifth step of forming a second semiconductor film on the barrier layer, the second semiconductor film containing a noble gas element with a concentration gradient set higher toward the film surface;
a sixth step of gettering to move the metal element into the second semiconductor film and to remove or reduce the metal element in the first semiconductor film having a crystal structure;
a seventh step of removing the second semiconductor film; and
an eighth step of removing the barrier layer.
In the above structures, the second semiconductor film may be formed by plasma CVD in which at least monosilane and noble gas are introduced as material gas into a film forming chamber to generate plasma, or by plasma CVD in which at least monosilane, noble gas, and hydrogen are introduced as material gas into a film forming chamber to generate plasma, or by plasma CVD in which at least monosilane, noble gas, and nitrogen are introduced as material gas into a film forming chamber to generate plasma, or by plasma CVD in which at least monosilane and phosphine are introduced as material gas into a film forming chamber to generate plasma.
In the above structures, it is preferable to remove impurities on the surface of the first semiconductor film before forming the barrier layer in order to reduce fluctuation even more.
In the above structures, a noble gas element may be introduced to generate plasma and change the surface condition of the first semiconductor film before forming the barrier layer in order to improve the adhesion.
In the above structures, a noble gas element may be introduced to generate plasma and change the surface condition of the barrier layer before forming the second semiconductor film in order to improve the adhesion.
In the above Structure 3, a noble gas element may be introduced to generate plasma and change the surface condition of the second semiconductor film before forming the third semiconductor film in order to improve the adhesion.
In the above Structure 2 or 3, the barrier layer is a silicon oxynitride film formed by plasma CVD in which silane-based gas and nitrogen oxide-based gas are introduced as material gas into a film forming chamber to generate plasma.
As has been described, the second semiconductor film containing a noble gas element and the barrier layer both can be formed by plasma CVD. Plasma CVD allows use of gas in cleaning of a film forming chamber (also called as a chamber). Therefore plasma CVD requires less maintenance than sputtering and is suitable for mass production.
In the above structures, the second semiconductor film containing a noble gas element and the barrier layer can be formed without exposing the device to the air. It is also possible to form the films in succession in the same chamber. Therefore the above structures are excellent in terms of throughput.
In the above structures, a method of manufacturing a semiconductor device is characterized in that the metal element is one or more kinds of elements selected from the group consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. A semiconductor film having an amorphous structure is crystallized well when doped with these metal elements.
In the above structures, the noble gas element is one or more kinds of elements selected from the group consisting of He, Ne, Ar, Kr, and Xe. When a semiconductor film contains these ions, dangling bonds and lattice strain may be formed to form a gettering site.
The present invention can provide a semiconductor film having a crystal structure in which a metal element for accelerating crystallization is sufficiently reduced or removed. Therefore electric characteristics of TFTs having the semiconductor film as their active layers are improved, in particular, OFF current is reduced, and fluctuation between elements can be reduced.